A Change of Seasons on Saturn

Looming like a giant flying saucer in our outer solar system, Saturn
puts on a show as the planet and its magnificent ring system nod
majestically over the course of its 29-year journey around the Sun.
These Hubble Space Telescope images, captured from 1996 to 2000, show
Saturn's rings open up from just past edge-on to nearly fully open as
it moves from autumn towards winter in its Northern Hemisphere.

Saturn's equator is tilted relative to its orbit by 27 degrees, very
similar to the 23-degree tilt of the Earth. As Saturn moves along its
orbit, first one hemisphere, then the other is tilted towards the Sun.
This cyclical change causes seasons on Saturn, just as the changing
orientation of Earth's tilt causes seasons on our planet. The first
image in this sequence, on the lower left, was taken soon after the
autumnal equinox in Saturn's Northern Hemisphere (which is the same as
the spring equinox in its Southern Hemisphere). By the final image in
the sequence, on the upper right, the tilt is nearing its extreme, or
winter solstice in the Northern Hemisphere (summer solstice in the
Southern Hemisphere).

Astronomers are studying this set of images to investigate the detailed
variations in the color and brightness of the rings. They hope to learn
more about the rings' composition, how they were formed, and how long
they might last. Saturn's rings are incredibly thin, with a thickness
of only about 30 feet (10 meters). The rings are made of dusty water ice,
in the form of boulder-sized and smaller chunks that gently collide with
each other as they orbit around Saturn. Saturn's gravitational field
constantly disrupts these ice chunks, keeping them spread out and
preventing them from combining to form a moon. The rings, as shown here,
have a slight pale reddish color due to the presence of organic material
mixed with the water ice.

Saturn is about 75,000 miles (120,000 km) across, and is flattened at
the poles because of its very rapid rotation. A day is only 10 hours
long on Saturn. Strong winds account for the horizontal bands in the
atmosphere of this giant gas planet. The delicate color variations in
the clouds are due to smog in the upper atmosphere, produced when
ultraviolet radiation from the Sun shines on methane gas. Deeper in
the atmosphere, the visible clouds and gases merge gradually into
hotter and denser gases, with no solid surface for visiting spacecraft
to land on.

The Cassini/Huygens spacecraft, launched from Earth in 1997, is well on
its way to the Saturn system. It will arrive in 2004 to land a probe on
Titan, Saturn's largest moon, and to orbit the planet for four years for
a detailed study of the entire Saturn system.

These images of Saturn, taken with the Wide Field Planetary Camera 2
onboard Hubble, were collected by Richard French (Wellesley College),
Jeff Cuzzi (NASA/Ames), Luke Dones (SwRI), and Jack Lissauer (NASA/Ames),
and have been prepared for presentation by the Hubble Heritage Team.